Rheostat



N0 18, 1930 N. c. scHELLENGER 1,781,959

BHEosTAT Filed Oct. 6, 1925 Inde/232377q Patented Nov. 18, 1930 UNITEDSTATES PATENT OFFICE NEWTON C. SCHELLENGER, 0F ELKHART, INDIANA,ASSIGNOR TO CHICAGO TELEPHONE SUPPLY COMPANY, OF ELKHART, INDIANA, ACORPORATION OF DELAWARE RHEOSTAT My invention 'relates to improvementsin the electrical art and more particularly to control apparatus .suchas rheostats and potentiometers.

I propose in general to provide a greatly improved rheostat which is notonly substantially moisture proof but is adapted to withstand severeservice and usage, such as is commonly experienced in radio apparatus.

In this rheostat of my invention, I employ an insulator strip renderedmoisture proof by a preservative such as linseed oil. This strip ispreferably treated in accordance with my novel process which includesimpregnating the strip with linseed oil and subsequently tumbling thestrip in a tumbler to remove surplus oil therefrom. The oil not onlyexpels the moisture from each strip but in displacing the moisture.fills the pores of the strip. Also after the strip has been treated anddried, the oil on the surface of the strip oX idizes thus forming' asmooth moisture proof protective coating for the strip.

The insulator strip when vassembled in the rheostat is positionedbetween the metallic frame or base and the resistance strip which ispreferably of the carbon type and is made in accordance with thefeatures of my copending patent application, Serial No. 139,784, tiledOctober 6th, 1926.

Also I propose to provide novel arcuate shaped clamp platesassociatedwith the ends of the resistance strip for clamping theresistance and insulator strips to the metallic frame. Each of theplates is provided -With a beveled edge for engagement with the contactroller of the contact arm and serves as a seat for the contact roller.

Now, in practice, I find that after an ordinary type of rheostat hasbeen in service for a period of time, and especially if the usage of thesame is severe, the contact arm will quite frequently fail to malte goodcontact with the resistance strip due either to wear or distortion ofthe parts. I propose to remedy this condition by providing a novelcontact arm assembly having resilient means'for at all times insuring agood contact between the arm and the resistance.

Irl-accordance with the general features oi Serial No. 138,783.

my invention, I provide'a cup shaped metallic frame or base in which isdisposed my novel contact arm assembly comprising a sliding arm havinga. slotted portion for receiving a collar connected to the operativeshaft of the rheostat and being provided with a bifurcated end forstraddling a portion of the contact roller which has a rolling contactwith the resistance strip. The sliding frame is resiliently connected tothe collar on the shaft by the means of a screw and a spring surroundingthe shank of the screw. This spring, as will be more fully described inthe accompanying specification, permits of the sliding frame movingrelative to the collar.

In accordance with other features of my invention, I furnish the cupshaped metallic frame or base with a transparent or Celluloid coverwhich is secured at its center by means of a screw to the end of therheostat shaft. The outer or peripheral edge of this cover is adapted toengage the external edge of the resistance and insulating strips and thesingle screw at 'the center of the Celluloid cover serves to not onlydraw the center of the cover into tight engagement with the end of theshaft but serves tol also draw the peripheral edge of the cover intotight engagement With the said strip. The cover also supports the end ofthe .operating shaft thus holding it true with the axis of the frame.Obviously' the purpose of the cover is to prevent dust and other foreignmatter from interfering with the proper operation of the contact roller.Also by substantially eliminating any dust from the interior of therheostat, it is possible to prevent change in the resistance of theresistance strip.

Other objects and advantages of my invenn tion will more fully appearfrom the followin;r detail description taken in connection with theaccompanying drawing, which illustrates one embodiment thereof and inwhich,

Figure 1 is a plan elevation of my no? rlieostat;

Fig. 2 is a perspective view ci the inswlat ing strip;

Fig. 3 is an enlarged sectional view of my rheostat taken onsubstantially the line 3 3 of F ig. 4; and

portion thimble is Fi 4 is a sectional view taken on substantially theline 4.--4 of Fig. 3.

Referring now to the drawing in detail, in which like reference numeralsdesignate similar parts throughout the several views, denotes generallyf a metallic frame or base which is preferably of cup shape. This framemay be made from sheet brass and is preferably a punch press product.Also if it is so desired, the frame may be buded and nickel-plated.

The cup-like frame or base 10 comprises a flat portion 11 and an annularrim 12. The port-ion 11 is provided with a central openingv 13 throughwhich extends the reduced 14 of a thimble 15. The axis of the disposedparallel to the axis of the frame or base 10. l-Also the thimble 15 withthe shoulder 16 sets tightly and squarely against'the frame portion 11.s

This thimble may be made on a screw .machine and is preferably made frombrass rod. It also may be nickel-plated. In order to secure the thimbleto the portion 11 of the frame '10, the reduced end 14 of the thimble 15is riveted at 17 to the portion 11 inside of the cup 10.

lExtending through the bore 18 of the l thimble 15 is a shaft- 20provided with a reduced portion .19 which projects into the interior ofthe cup-like frame 10. The shaft 20 `has an annular shoulder 21'whichengages an annularshoulder 22 formed in the thimble 15. These twoshoulders 21 and 22 cooperate to prevent movement of the shaft 20 in onedirection. u

Also the thimble 15 is provided with an 1external. threaded portion 23projecting through an annular opening 24 in a mounting anel 25. Theframe 10 isftightly clamped to the panel 25 by means of a nut 26threaded ont-o the portion 23 of the thimble 15 (Fig. 4). A thin washer27 is preferably positioned between the nut 26 and the base of theIpanel 25. Fastened to one end of the shaft 2O is a knob 28 which'includes a metallic thimble 29. The knob and thimble are se- .cured tothe shaft by means of a set screw 30 extending through an aperture 31formed in the knob 28. It will be observed from Fig. 4 that the knob 28includes an annular `iange 33 for substantially enclosing the part usedto fastenthe frame to the panel 25.V

AAlso the knob'33 may be made of any suitable insulation, such, .forexample, as molded phenolic condensation matter.

Surrounding the reduced ortion 19 of A the shaft 20 is a collar 35,which has an exltension 36 telescoping a washer 37. The extension 36istightly iitted into the washer orring-li-ke member 37 and has anannular lateral -liange 38 whichl serves to secure the 'member 37 to thecollar 35. Both the collar H' and the member 37as well as the shaft 2Omy bemade ffrorn'vsuitable brass rod. These parts may also be providedwith a mechanical nickel-plate plating. The collar 35 is providedv witha pair of cut-awayportions 39 and 40 for receiving the legs 41 and 42 ofa slidable frame 44. This frame 44, as will be more fully describedhereinafter, is adapted to be moved relative to the collar 35.

Legs 41 and 42 of the frame 44 are connected at one end by means of alateral extension piece 43. This extension piece 43 has an opening 45through which the shank of a screw 46 extends. Disposed about the shankof the screw 46 is a coil spring 47 preferably of the compression type.One end of this spring abuts the head of the screw 46 and the other endabuts the extension piece 43. This screw 46 is threaded at 48 into thecollar 35. Also it should be noted l i erably made of brass rod and inreality comprises two rolling portions 53 and 54 connected by thereduced ortion 51. Sliding frame 44 may be made rom relatively hardbrass sheet. Both the frame and the roller are nickel-plated.

Located between the portion 11 of frame 10 and the flange 38 of theextension 36 and surroundin the reduced portion 19 is a spring was er55. This washer serves to urge the contact arm assembly in a directionaway from the panel 25. I have designated the contact arm assemblygenerally by the reference character 56. It will be evident that thisassembly 56 which includes collar 35, sliding frame or arm 44 and roller52, cancbe removed with facility from the shaft 20 by releasing thescrew 46 from engage ment with the reduced portion 19. e

vPositioned. within the cup-like frame or base 10 and abutting theinnerperiphery of the side 12 is my novel insulating strip 601 I shalldescribe more in detail the process of making the strip hereinafter.This strip 60 is bent into an arcuate shape when it is mounted in theframe. A resistance strip 61 is also disposed-within the cup-like frame10. This strip is bent into an arcuate shape and is contiguous with theinsulating str1p 60 which serves to insulate the resistance strip 61from the frame 1 0.

Resistance strip 61 is made in accordance with the features of my"copending patent application SerialNo. 139,784, filed October 6th,1926. Briefly, this strip consists of a fibrous sheet of material havinga coating of my novel resistance paint thereon. The resistance paint orcoating consists of a Imixture of. graphite, lamp black and gumreference character 80.

shellac. In this mixture, I preferably employ 8 pennyweight of lampblack and 7 pennyweight of graphite. These carbons are mixed in a gumshellac solution of from 7 to 12 iiuid ounces .depending upon whatresistance is desired. `The shellac is preferably diluted with alcoholuntil it gives a mixture which will read approximately 29 on a Baumeammonia hydrometer scale. The carbon paint may be applied to the sheetof fibrous material with a suitable varnish brush and then dried foragiven period of time at a temperature of 170O Fahrenheit. Theadvantages of this resistance are fully set out in my copending patentapplication.

The strip 6l is insulated from the flat portion 11 of the frame 10 bymeans of an annular gasket or washer 63. This gasket 63 ma be made ofany suitable insulatin material such, for example, as press board.eferring to Fig. 3, it will be obvious that one end of the strip 60 andthe corresponding en?. of the strip 61 are secured to the ange 12 of theframe 10 by means of a screw 65 and a pair of lock nuts 66 and 67. Awasher 68 is disposed between the screw 67 and the outer peripher of theportion 12. An aperture 69 is provi ed in the frame 10 for accommodatingthe shank of screw 65. Also the screw 65 extends through suitableapertures or openings in the associated ends of the strips 60 and 61.

The other end of the strip 60 and the associated end of the resistancestrip 61 are secured to the frame by means of a screw 70. This screwextends through an enlarged opening 71 in the annular flange 12 of theframe 10. A bushing 72 is disposed in the enlarged opening 71 and servesto insulate the shank of screw from the frame 10. Screw 70 is locked inplace by means of a pair of lock nuts 73 and 74. A pair of wash-` ers 75and 76 are disposed between nut 74 and the flange 12. Washer 75 ispreferably made of metal and washer 76 must be made of insulatingmaterial so as to insulate the screw from the frame.

In Fig. 2, I have illustrated my novel insulating strip 60. This strip60 has a pair of apertures or o enings 78 or receiving the screws 65 and70 respectively, as previously described. The resistance strip 61 has apair of similar lopenings adapted to be aligned with openings 78. Ishall now proceed to describe in detail my novel clamping platesassociated with the heads of the two screws 65 and 70.

I designate the clamping plates by the Since these two plates areidentical as to structure, it is believed the description of lone willYsuffice for both. Each -of the plates is arcuate in shape so as toaccommodate itself to the arcuate shaped strips associated therewith.Also each plate has an aperture 81 through which sheets.

the shank of the associated screw extends. Each ofthe plates has aground beveled ed e 82 adapted to be engaged by the roller 52. In orderto insure a good electrical contact between the roller and each of theplates, I preferably make the plates from nickel silver These plates notonly serve as clamps for securing the strips to the frame but alsocooperate with the screw heads in limiting the travel of the cont'actroller. The heads of screws 65 and 70 actually serve as the stops forlimiting the travel of the contact roller.

. The end of the portion 19 of shaft 20 is reduced at 85 so as to fit inopening 86 in a cover 87. The cover 87 is a circular plate ofinsulation, and is preferably made of transparent material such aslCelluloid. The pey ripheral edge of this cover is adapted to engage theoutermost edges of the two strips 60 and 6l, as is obvious in Fi 4. Alsothe portion 1 9 of shaft 20 is provi ed with a threaded opening 88 forreceiving the threaded shank of a screw 90. The screw 90 serves to drawthe Celluloid cover 87 into tight engagement with the shoulder 91 on theshaft 20. Cover 87 not only protects the contact mechanism from dust andforeign matter but also protects the parts from moisture. n

I shall now describe in detail the process of manufacturing my novelinsulating strips 60. Thestrips 60 are preferably punched press boardand are each provided with the previously mentioned pair of perforations78. Each strip is first boiled in a suitable oil such, for example, aslinseed oil. This boiling operation results in themoisture beingexpelled from the strips and the oil in displacing the moisture fillsthe pores of the strips. After the strip has been tumbled and dried,itwill be found that the oil lhas oxidized thus providing a waterproofcoating for the strip.

In order to remove the surplus oil from the strips, each strip is placedalong with others immediately after boiling in a tumbling barrel andtumbled. This tumbling action not only removes the surplus oil from thestrip, but also polishes its surface.

The roller 52 is adapted to exert substantially a one pound pressure onthe resistance strip; this insuresa Vvery positive electrical connectionbetween the roller and the resistance. By adjusting the screw 46, thetension of spring 47 can be varied thus enabling the pressure oftheroller to be properly adjusted at all times.

The operation of my novel rheostat is believed to be obvious from theforegoing detail description of the same. By turning the knob onlshaft20, the roller 52 can be made to roll along the surface of theresistance strip 61 to varyv the resistance. Any irregularities in themovement of the roller 52 are compensated for by the spring whichpermits the frame 44 to move relative to the collar 35.

I desire it understood that my invention is not to be limited by theabove foregoing description but only insofar as defined by the scope andspirit of the appended claims.

I claim 1. In an electrical current control device, a cup shaped body, aresistance element disposed in said cup shaped body, movable contactmeans associated therewith and a clamping plate for securing theresistance element to the body, said plate having a beveled edgedisposed in the path of movement of said lcontact means and havingtracking cooperat'ion therewith.

'2. In an electrical current control device, an arcuate shaped cupconstituting the frame ofthe device, a resistance disposed in the saidcup, contact mechanism associated therewith including a contact rolleradapted to cooperate with the said resistance, and an arcuate shapedclamping plate for fastening an end of the said resistance to the saidcup, said plate having a beveled edge disposed in the path of movementof and cooperable with the roller. v

3. In an electrical current control device, a metallic frame comprisinga cup shaped member,.a resistance strip disposedin the said member, aninsulating strip for insulating the resistance from the frame, contactmechanism connected to the frame adapted to cooperate with the saidresistance strip, said mechanism including a roller adapted to have arolling engagement with the resistance strip, and means for clamping theends of the Said strip to the frame, said means including a pair of*clamping plates, each of the said plates bein arcuate in shape andhaving a beveled edge isposed in the path of movement of and havingtracking cooperation with the roller.

4. In an electrical current control device, a cup-like base, aresistance element disposed in the said base, contact mechanismpositioned in the cup base adapted to cooperate with the said resistanceelement, said mechanism including a resiliently mounted roller adaptedto have frictional enga ement with the said resistance element, 'anmeans for fastening the resistance element to the base comprising a stopfor limiting the travel of the said roller and a beveled edged clampingplate drawn upon the resistance element by said stop and tracking theroller into cooperation with the sto 5. In an electrical current controldevice having an operating rod and a resistance element, the combinationof a collar connected to the said rod, a slidable element associatedwith the said collar, an adjusting screw for connecting the said elementto the collar, resilient means 4carried by the said screw for permittingthe said slidable element to move Lvsnese relative to the collar, and aroller pivotally connected to the said slidable element adapted to havefrictional engagement with the said resistance element.

6. In a device of the class described, the combination of a resistanceelement, a rotatable operating member having a splayed portion, a yokedmember straddling the splayed portion of said operating member andhaving movement radially of the operating inember in a planeintersecting the axis thereof and contact means cooperable with saidresistance element and associated with and operated by said yokedmember.

7. In a device ofthe class described, the combination of a resistanceelement, a rotatable operating member having a splayed portion,a yokedmember straddling the splayed portion of said operating member andhaving movement radially of the operating member in a plane intersectingthe axis thereof, contact means cooperable with said resistance Velementand associated with and operated by said yoked member and meanscooperating with said yoked member and yieldingly urging said contactmeans into contact with said resistance element.

8. In a device of the class described, the combination of a resistanceelement, a rotatable operating member having a splayed portion, a yokedmember straddling the splayed portion of said operating member andhaving movement radially of the operating member in a plane intersectingthe axis thereof, contact means cooperable with said resistance elementand associated with and operated by said yoked member, an abutment and aspring interposed between said abutment and said yoked member andyieldingly urging said contact means into contact with said resistanceelement.

9. In an electric current control device having an operating rod andaresistance element, the combination of a collar, an arm movable withrespect to the collar, a contact member carried by the arm, a screwrigidly securing the collar to the operating rod and a spring carried bythe screw for yieldingly urging the contact member into contact with theresistance element.

10. In combination, an operating rod, a collar on the rod, an armmovable with respect to the collar, a headed screw rigidly securing thecollar to the rod, a lug on the arm, and a spring disposed between thelug and the head of the screw.

v11. In combination, an operating rod, a

slotted collar on the rod, an army disposed in the slot in the collarand slidable therein, a headed screw rigidly securing the collar to therod, a lug on the arm, and a spring disposed between the lug and thehead of the screw for yieldingly urging the arm away from the head ofthe screw.

12. In combination, an operating rod, a col-

